ANIMALS AND THEIR ENVIRONMENT 287 



approaches more of the water is adsorbed on the colloidal (page 42) 

 particles in the pupal liquids, leaving less water free. As a consequence 

 of this condition, the pupa endures winter freezing for months. 



Nutrition. — With very few exceptions, all of which are among the 

 protozoa, animals are ultimately dependent on plants for their food. The 

 green plants provide carbohydrates by photosynthesis, and a few micro- 

 organisms; including those forming nodules on the roots of clover and 

 other legumes, can utilize the nitrogen of the air to produce nitrites and 

 nitrates. Out of these primary substances animals can make any com- 

 pounds they require, but plants have to make the start. 



The manner of taking foods from plants is very variable. Many 

 insects or their larvae eat the leaves or suck sap from the leaves, stems, 

 or roots. Some eat the wood, though it is quite possible that fungi or 

 other organisms growing on the wood or in the burrows form part of their 

 food. Bees get carbohydrates (honey) from the flowers and proteins 

 from pollen. Many animals grow on decaying logs or other plant matter, 

 but it is likely that the microorganisms which are always present in 

 such places constitute the actual food. Of the animals that do not feed 

 directly upon plants but upon other animals, the larger ones usually, 

 and the small ones often, kill their prey and eat its flesh. The larvae 

 of the clothes moth eat hair or wool. Some insects live in the excrement 

 of animals, but here again it is probably the microorganisms that furnish 

 the food. 



A very special way of obtaining nutrition is through parasitism. 

 The host is usually not killed — -at least until the parasite is past its para- 

 sitic stage — but contributes some of its substance to the parasite. The 

 flukes and tapeworms are regularly parasitic, as are some of the round- 

 worms and some insects. Parasites show a tendency to be degenerate, 

 which they can afford to be, since in their protected situations and with 

 their food often digested (page 265) before they receive it many of the 

 usual organs are unnecessary. The advantages of parasitism accrue only 

 to the parasite, none to the host. 



Contrasted with this is the relation known as symbiosis, which is an 

 association of two species with mutual benefit. A very striking example 

 of symbiosis, in which food appears to be at least part of the advantage 

 gained by both species, is that existing between termites (the so-called 

 white "ants") and certain protozoa harbored in their intestines. The 

 protozoa are so abundant that in some instances they weigh as much 

 as the termite itself. The termites are wood-eating insects, and their 

 normal food is cellulose. They are not themselves, however, able to 

 digest the cellulose. This is done for them by the protozoa (page 259). 

 These protozoa may be removed from the intestine artificially by high 



